1. Field of the Invention
The present invention relates to an engine unit for a straddle-type vehicle.
2. Description of Related Art
Generally, an engine for a straddle-type vehicle such as a motorcycle comprises a clutch mechanism that transmits rotation of a crank shaft to an output shaft, or cuts off the rotation. An automatic centrifugal type clutch mechanism has a bowl-shaped outer clutch, of which an axial end is opened, fixed to an output side thereof and an input member arranged in the outer clutch to fix the same to an input side thereof (see, for example, JP-A-2001-3723). There is also known a wet multi-disc type clutch mechanism having a multiplicity of inner and outer clutch plates interposed between an outer clutch fixed to an input side thereof and an inner clutch fixed to an output side thereof (see, for example, JP-B-6-65894).
In such wet type clutch mechanisms lubricating oil must be supplied to inhibit seizure between constituent members of the clutch mechanism, such as between an outer clutch and an input member, between an inner clutch plate and an outer clutch plate, or the like. In one method of supplying lubricating oil, there is no specific construction for ensuring lubrication of the clutch mechanism, and the clutch mechanism itself scatters lubricating oil collected on a bottom of a crank case to provide for lubrication. However, there is a concern that a sufficient quantity of lubricating oil may not surely be supplied to the clutch mechanism only by the clutch mechanism itself scattering the lubricating oil.
A method has also been proposed that uses an exclusive lubricating-oil pump to supply lubricating oil to a clutch mechanism. While this method surely supplies lubricating oil to the clutch mechanism, size and complexity of the clutch mechanism is increased due to the need for an exclusive lubricating-oil pump.
In view of this problem, WO2003/085278 discloses a method by which lubricating oil supplied to a connection of a crank pin and a connecting rod is led to the clutch mechanism.
FIG. 5 is a cross sectional view showing an engine unit 112 described in WO2003/085278. As shown in FIG. 5, an oil supply passage 120d is formed in a crank shaft 120. Lubricating oil is supplied to a connection 142 of a crank pin 115, which is mounted to crank arms 120a, 120b, and a connecting rod 124 via oil supply passage 120d. Lubricating oil as supplied scatters rearward upon rotation of crank shaft 120. As shown in FIG. 6, an open hole 117c is formed in a clutch chamber and the scattering lubricating oil is introduced into the clutch chamber from open hole 117c. Lubricating oil as thus introduced is supplied to a clutch shaft 140 and a clutch mechanism 150.
With the engine unit of WO2003/085278, however, a transmission shaft 116a is arranged between crank shaft 120 and clutch shaft 140 and prevents lubricating oil scattered by rotation of crank shaft 120 from being supplied to clutch shaft 140. Accordingly, there is a problem in that it is difficult to efficiently supply lubricating oil to clutch shaft 140 and clutch mechanism 150.